Scotch-Irish Americans are American descendants of primarily Ulster Scots people, who emigrated from Ulster (Ireland's northernmost province) to the United States between the 18th and 19th centuries, with their ancestors having originally migrated to Ulster, mainly from the Scottish Lowlands and Northern England in the 17th century.

In the 2017 American Community Survey, 5.39 million (1.7% of the population) reported Scottish ancestry, an additional 3 million (0.9% of the population) identified more specifically with Scotch-Irish ancestry, and many people who claim "American ancestry" may actually be of Scotch-Irish ancestry.

The term Scotch-Irish is used primarily in the United States, with people in Great Britain or Ireland who are of a similar ancestry identifying as Ulster Scots people. Many left for North America, but over 100,000 Scottish Presbyterians still lived in Ulster in 1800. With the enforcement of Queen Anne's 1704 Popery Act, which caused further discrimination against all who did not participate in the established church, considerable numbers of Ulster-Scots migrated to the colonies in British America throughout the 18th and 19th centuries.

The term is first known to have been used for Scottish Catholics in Ireland. In a letter of April 14, 1573, in reference to descendants of "gallowglass" mercenaries from Scotland who had settled in Ireland, Elizabeth I of England wrote:

We are given to understand that a nobleman named Sorley Boy MacDonnell and others, who be of the Scotch-Irish race ...

This term continued in usage for over a century before the earliest known American reference appeared in a Maryland affidavit in 1689–90.^{[citation needed]}

Scotch-Irish, according to James Leyburn, "is an Americanism, generally unknown in Scotland and Ireland, and rarely used by British historians". It became common in the United States after 1850. The term is somewhat ambiguous because some of the Scotch-Irish have little or no Scottish or Irish ancestry at all: numerous dissenter families had also been transplanted to Ulster from northern England, in particular the border counties of Northumberland, Cumberland, Durham, and Yorkshire. Smaller numbers of migrants also came from Wales, the Isle of Man, and the southeast of England, and others were Protestant religious refugees from Flanders, the German Palatinate, and France (such as the French Huguenot ancestors of Davy Crockett). What united these different national groups was a base of Calvinist religious beliefs, and their separation from the established church (the Church of England and Church of Ireland in this case). That said, the large ethnic Scottish element in the Plantation of Ulster gave the settlements a "Scottish character" rather than an Irish one.

Upon arrival in North America, these migrants at first usually identified simply as Irish, without the qualifier Scotch. It was not until a century later, following the surge in Irish immigration after the Great Irish Famine of the 1840s, that the descendants of the earlier arrivals began to commonly call themselves "Scotch-Irish" to distinguish themselves from the newer, poor, predominantly Catholic immigrants. At first, the two groups had little interaction in America, as the Scots-Irish had become settled many decades earlier, primarily in the backcountry of the Appalachian region. The new wave of Catholic Irish settled primarily in port cities such as Boston, New York, Charleston, Chicago, Memphis and New Orleans, where large immigrant communities formed and there were an increasing number of jobs. Many of the new Irish migrants also went to the interior in the 19th century, attracted to jobs on large-scale infrastructure projects such as canals and railroads.